Embodiments described herein relate to the field of airborne electromagnetic surveying.
Electromagnetic (“EM”) sensing equipment is routinely used to perform EM surveying for geophysical mapping purposes. Two types of EM surveying include passive source EM surveying and active source EM surveying. In passive source EM surveying, a magnetic receiver is used to measure a magnetic response of the earth (“earth response”) to naturally occurring sources of stimuli such as lightening strikes. In active source EM geological mapping, a magnetic receiver is used to measure the earth response to a primary magnetic field (“primary field”) transmitted by the survey system. The relation between the transmitted primary field and the earth response is used to calculate the electrical resistivity structure of the earth, from which geological information is inferred.
Active EM surveying includes both frequency domain EM (FDEM) and time domain EM (TDEM) techniques. In FDEM, the earth response is measured as a function of frequency. In TDEM, the earth response is measured as a function of time after a transmitted pulse. In all FDEM and TDEM techniques, the magnetic receiver senses the superimposed primary field and earth response.
EM surveying is implemented using ground, water, and airborne equipment. Airborne systems exist for both fixed wing and helicopter aircraft. In the case of helicopter borne EM systems, the receiver and transmitter may be attached to a structure (“bird”) which is towed by the helicopter. In some systems the receiver is located close to the receiver and the primary field magnitude is large relative to the earth response.
In a practical active EM survey system, a time varying electric current (“transmitter current”) is passed through a wire coil or loop (“transmitter loop”), thereby generating a time varying magnetic field (“primary field”). The magnetic field induces electric currents in the earth, which generate a secondary magnetic field, the earth response. The earth response is sensed by one or more receivers and is recorded by a data acquisition system. The receiver may itself be a wire coil or loop (“receiver coil”), although other types of receivers have been used.
In active source EM systems, the magnitude of the earth response can be orders of magnitude less than the effect of the primary magnetic field on the receiver. In passive source EM systems, the earth response that must be measured can be of very low magnitude. Accordingly, in both systems noise mitigation is a primary concern and accurate measurement of the earth response typically involves a combination of physical equipment and electronic signal processing to both reduce the impact of noise on the receiver itself and also to remove noise from any signals measured by the receiver. Examples of systems that strive to provide accurate earth response measurements are described, for example, in U.S. Pat. Nos. 8,674,701 and 6,876,202, which are directed to an active system and a passive system respectively.
Despite the extensive advancements that have been made in the field of airborne surveying, achieving further advancements in noise mitigation is desirable.